Field of the Invention
This invention relates generally to the fields of parasitology, malaria research, and malaria vaccine development. More particularly it relates to Plasmodium sporozoites of human host range and the in vitro culturing of the mosquito stages of infectious Plasmodium parasites of human host range, particularly to sporozoite stage, and the use of in vitro cultured Plasmodium sporozoites as an immunogenic component of vaccines and other reagents.
Background of the Invention
Annually, Plasmodium falciparum (N) malaria causes >200 million clinical cases, more than 600,000 deaths, and is responsible for loss of greater than $12 B of Gross Domestic Product in Africa [1-3]. Malaria is also a serious concern for travelers and military personnel. During 2010-2011 the number of cases of malaria in travelers from the United Kingdom increased by 30% [4]. In 2011, the U.S. had more cases of malaria than in any year in the past 40 years [5, 6]. In all U.S. military campaigns in highly malarious areas during the past 150 years, U.S. forces have had more casualties from malaria than from hostile fire [7]. A highly effective vaccine will have a dramatic impact on the roughly 2.5 billion “at-risk” individuals in the Global Health market.
The world community is now spending approximately $2 billion annually to control malaria through use of insecticide-impregnated bednets, insecticides, and antimalarial drugs. This amounts to approximately $80 per year for every child born in Africa, and in some locations 5 to 10 times that amount is being spent. These approaches are having an excellent effect in many areas. However, drug and insecticide resistance is still developing, and the ability of financial donors and local governments to sustain this effort is limited. It is clear that elimination of malaria from high transmission areas will require new tools. As described in a 2010 editorial, a highly effective vaccine would be the ideal tool for prevention, control and elimination of malaria worldwide:                “What is still needed is the only tool that has ever truly conquered any infectious disease: an effective . . . affordable vaccine . . . here, the global malaria community has been too complacent . . . GlaxoSmithKline's . . . RTS,S plus adjuvant AS01 is a first-generation pre-erythrocyte-stage vaccine with modest and time-limited efficacy . . . We cannot afford to wait a further 20 years for the next generation . . . vaccines . . . .”        Anonymous, The Lancet, Apr. 24, 2010        
And as described in a 2011 ma1ERA initiative report, the ideal vaccine would be a pre-erythrocytic-stage vaccine that prevents parasites from getting out of the liver into the bloodstream, thereby preventing disease as well as transmission [8]. This has been termed a “vaccine that interrupts malaria transmission” (‘VIMT’).
Glaxo Smith Kline has developed a vaccine candidate termed RTS,S/AS01, which uses a recombinant protein (that fuses part of the Pf circumsporozoite protein (CSP) with hepatitis B surface antigen) with a strong adjuvant (AS01). Recent Phase 3 trials [9-12] in 5-27 month old humans demonstrated a 36% reduction in the incidence of malaria during a year and a 56% reduction in the rate at which malaria was acquired during the first year, and a 47% reduction in severe malaria during the first year. Unfortunately, the results in infants were not as strong. In 6-12 week old humans, the vaccine demonstrated a 16% reduction in the incidence of malaria during a year, a 31% reduction in the rate at which malaria was acquired during the first year, and a 36% reduction in severe malaria (26% by intention to treat) during the first year. These results have been called disappointing and would not qualify this vaccine as highly effective or as a VIMT.
During the last ten years, the focus for the development of a highly effective VIMT malaria vaccine has shifted in part to the utilization of the whole parasite, sporozoite (SPZ) stage, of Plasmodium as the vaccine immunogen. In a recently completed study at the Vaccine Research Center (VRC) at National Institute of Allergy and Infectious Disease (NIAID), the Sanaria® PfSPZ Vaccine, composed of radiation attenuated Pf SPZ, was administered by intravenous (IV) injection and protected 6 of 6 (100%) of the volunteers who received the highest dose. There was a dose response in regard to protective efficacy (6/9 protected at next lower total dose) and a significant correlation between titers of antibodies against Pf SPZ and protection. Sanaria® PfSPZ Vaccine is therefore demonstrably potent and highly protective in humans. These historic results were published online in Science in August 2013 and in print in September 2013 [13].
SPZ are also being used as the parasite component of an infection and treatment approach to vaccination called Sanaria® PfSPZ-CVac, in which live infectious Plasmodium SPZ are administered in the presence of an asexual erythrocytic stage anti-malarial such as chloroquine [14].
Finally, live infectious Pf SPZ are being used for controlled human malaria infections (CHMI) as a means for testing malaria vaccines and other therapeutics [15, 16].
Substantially purified Plasmodium sporozoites prepared from salivary glands extracted from mosquitoes and grown in culture are described in U.S. Pat. No. 8,043,625, which is incorporated herein by reference.
Presently, the whole parasite Pf SPZ used in the vaccines and reagents described above have been obtained by rearing aseptic Anopheles mosquitoes, infecting them with aseptic Pf gametocytes, permitting the Pf parasites to progress through sporogony in vivo within the mosquito, to the sporozoite stage, and then hand dissecting the salivary glands from the mosquitoes and isolating and purifying the aseptic sporozoites (U.S. Pat. Nos. 7,229,627; 8,367,810) [17]. While this manufacturing approach is capable of producing sufficient quantities of live, aseptic purified Pf SPZ, for use in all the clinical trials for these products, the methodology is labor intensive and requires substantial resources for insect husbandry and parasite dissection. In particular, dissecting from the mosquito salivary glands is a technical and time-consuming step in the production of Pf SPZ and other Plasmodium-species SPZ of human host range.
The mosquito host stages of Plasmodium parasite development are shown in FIG. 1. While efforts to establish the asexual portion (vertebrate-host stages) of the Plasmodium life cycle in vitro have been successful [18] substantial effort has been made to accomplish the same for the sexual (mosquito-host stages) and sporogonic portion, but these efforts have been unsuccessful for producing clinically relevant infectious Plasmodium sporozoites of human host range, particularly Pf SPZ. In vitro transformation of P. gallinaceum (avian host range) and Pf ookinetes resulted in low numbers of oocysts and SPZ, but infectivity of these sporozoites was never demonstrated [19-20]. In vitro transformation of P. berghei (rodent host range) produced oocysts and SPZ, but the SPZ were much less infective than were mosquito-derived SPZ [21].